The present invention relates generally to LED driver circuits for driving LED lighting devices formed by an array of light-emitting diodes. More particularly, the present invention relates to an LED driver circuit effective to prevent continuous driving in an open load state such as when the light-emitting diodes are removed or otherwise disconnected from the output terminals of the LED driver circuit to form an open circuit.
An example of an LED driver circuit as previously known in the art (and as represented in FIG. 2) is formed by an AC power source AC, a diode bridge DB, a DC/DC converter 1, an LED lighting device 2, and a control circuit 3.
The AC power source AC is a commercial power source. In this conventional example, the AC power source AC outputs an AC voltage V1 of 100 V to the diode bridge DB. The diode bridge DB functions as a full-wave rectifying circuit, and a capacitor C1, such as an electrolytic capacitor, is coupled across output ends of the diode bridge DB. The diode bridge DB rectifies the AC voltage V1 so as to generate a DC voltage V2a of substantially 140 V across the capacitor C1.
The DC/DC converter 1 is a step-down chopper (buck converter) circuit with a diode D1, a capacitor C2, an inductor L1, and a switching element Q1. The capacitor C2 may be an electrolytic capacitor and the switching element Q1 may be an n-type channel MOSFET.
A series circuit of the capacitor C2, the inductor L1, the switching element Q1, and a resistor R1 is coupled in parallel with the capacitor C1. A positive electrode of capacitor C2 is coupled to a positive electrode of capacitor C1, and a negative electrode of capacitor C2 is connected to inductor L1. The drain terminal of switching element Q1 is coupled to inductor L1, the gate terminal is coupled to the control circuit 3, and the source terminal is coupled to a negative electrode of capacitor C1 via resistor R1. The diode D1 is coupled in parallel with capacitor C2 and inductor L1. The anode terminal of diode D1 is coupled to a node between inductor L1 and switching element Q1. The cathode terminal is coupled to the positive electrode of capacitor C2.
In the DC/DC converter 1 (buck converter) having the above described configuration, the switching element Q1 is turned ON/OFF by the control circuit 3 so that the AC voltage V2a is converted to a DC voltage V3 across the capacitor C2.
The LED lighting device 2 includes a plurality of light-emitting diodes, and is coupled in parallel to capacitor C2. A driving current through the LED lighting device 2 is caused to flow in accordance with the DC voltage V3 generated across the capacitor C2.
The control circuit 3 has a DC power source E as an input power source, and controls switching element Q1 of the DC/DC converter 1 so that a predetermined LED current flows through the LED lighting device 2. The control circuit 3 detects current flowing through switching element Q1 by detecting a voltage across resistor R1. The control circuit 3 also detects current flowing through inductor L1. When the current flowing through switching element Q1 exceeds a predetermined value, the control circuit 3 turns OFF switching element Q1. When the current flowing through inductor L1 falls below a predetermined value, the control circuit 3 turns ON switching element Q1. Thereby, the control circuit 3 controls the current flowing through the LED lighting device 2 to be substantially constant (i.e., a constant current control operation).
An open circuit state wherein the LED lighting device 2 is not coupled to the LED driver circuit (i.e., an open circuit across output terminals of the LED driver circuit) will now be described. When the LED lighting device 2 is not connected, current does not flow through switching element Q1 and control circuit 3 keeps driving switching element Q1 in an ON state. The negative electrode of capacitor C1 and the negative electrode of capacitor C2 are thereby at substantially the same potential, and DC voltage V2 is applied across capacitor C2. Therefore, in the LED driver circuit of the conventional example as shown in FIG. 2, it is necessary to select a high voltage electrolytic capacitor as a capacitor C3 to be used in the output end of the DC/DC converter, which results in an undesirable increase in cost.